1. Field of the Invention
The present invention relates to an apparatus and a method for estimating tire air pressure, and specifically to an apparatus and a method of estimating tire air pressure based on a wheel speed signal.
2. Description of the Related Art
Conventionally, as an apparatus for estimating a state of tire air pressure state, there is a known technology of sampling tire resonance frequency tire by subjecting the wheel speed signal, including frequency components of tire vibration in an operating a vehicle to frequency analysis and detecting tire air pressure state based on the resonance frequency (for example, Japanese Patent Registration No. 2,836,652 and the like).
Resonance frequency within the scope of technology for estimating air pressure in this way, generally falls in a range of about 30 through 50 Hz. However, according to the technology, there is a disadvantage in that there is at least one situation in which tire air pressure cannot be estimated accurately depending on the environment to which the vehicle is subjected as shown below.
FIGS. 9A and 9B show an example of a measurement result of a power spectral level with respect to wheel speed signal. Further, FIG. 9A shows an example of a situation in which vehicle speed is in a mid-range speed of xe2x80x9caxe2x80x9d km/h and FIG. 9B shows an example of a situation in which the vehicle speed is in a high range speed of xe2x80x9cbxe2x80x9d km/h. As shown by the drawings, in a range of a resonance frequency of about 30 through 50 Hz, when the vehicle speed is slow (FIG. 9A), the power spectral level (gain) of the resonance frequency (coupled primary resonance frequency), is high and accordingly, the resonance frequency can be accurately sampled. However, when the vehicle speed is fast (FIG. 9B), the power spectral level of the resonance frequency is low and accordingly, accurate resonance frequency cannot be accurately sampled. This is caused by the fact that when the vehicle is in a high speed region, tire vibration phenomenon has difficulty in occurring.
Therefore, for example, when the vehicle is operating in low to mid speed regions in an urban area, tire air pressure can be estimated with high accuracy in the above-described range of resonance frequency of about 30 through 50 Hz. However, when the vehicle is operating in a high speed region, the power spectral level of the resonance frequency decreases and accuracy for estimating tire air pressure deteriorates.
In order to address this disadvantage, according to technology described in Japanese Patent Application Laid-Open (JP-A) No. 9-2031, there technology proposed for estimating tire air pressure with high accuracy regardless of the vehicle speed by estimating tire air pressure based on the spring constant constituting an index having a high correlation with the resonance frequency when the vehicle speed is slow and estimating tire air pressure based on tire dynamic load radius when the vehicle speed is fast.
According to the technology described in JP-A No. 9-2031, as a condition of switching between the two methods of estimating air pressure, the travel speed of the vehicle is applied. However, the threshold for switching between estimating methods in this case is a fixed determination based on analyzing the vehicle speed when the power spectral level is lowered by experiments or the like. Moreover, the threshold is determined for each of various elements of the vehicle (particularly, tire type). Accordingly, there is a disadvantage in that although estimation accuracy is high for a vehicle corresponding to various elements for which the appropriate threshold has been predetermined, estimation accuracy is low for other vehicles.
The invention addresses the above-described disadvantage and it is an object thereof to provide an apparatus for estimating tire air pressure with higher accuracy from operation at low to high speeds regardless of the various elements mounted to the vehicle.
In order to achieve the above-described, according to a first preferred embodiment of the invention, there is provided a tire air pressure estimating apparatus comprising an extracting component for extracting a resonance frequency or a spring constant of a tire based on a wheel speed signal including frequency components of vibration of the tire in operating a vehicle, a first estimating component for estimating tire air pressure based on the resonance frequency or the spring constant of the tire extracted by the extracting component, a deriving component for deriving a dynamic load radius of the tire based on the wheel speed signal, a second estimating component for estimating the tire air pressure based on the dynamic load radius derived by the deriving component, and a switching component for selectively switching a estimation of the air pressure by the first estimating component in accordance with a statistic value based on the resonance frequency or the spring constant of the tire at each of a plurality of time points.
The extracting component extracts the resonance frequency or the spring constant of the tire based on the wheel speed signal including the frequency components of vibration of the tire in operating the vehicle. The tire air pressure is estimated by the first estimating component based on the extracted resonance frequency or the extracted spring constant of the tire. As a method of estimating the tire air pressure by the first estimating component, there can be applied all existing methods capable of estimating the tire air pressure based on the resonance frequency or the spring constant of the tire which have estimating methods described in previous publications such as, for example, Japanese Patent Registration No. 2,836,652, JP-A No. 9-2,031, JP-A No. 6-297,923 and JP-A No. 8-219,920.
According to the first embodiment of the invention, the dynamic load radius of the tire is derived based on the wheel speed signal by the deriving component and the tire air pressure is estimated by the second estimating component based on the derived dynamic load radius. As a method of estimating the tire air pressure by the second estimating component, there can be applied all the existing methods capable of estimating the tire air pressure based on the dynamic load radius of the tire such as, for example, a estimating method described in JP-A No. 9-2,031.
Further, according to the first embodiment of the invention, selective switching of estimation of the air pressure by the first estimating component and the second estimating component is performed in accordance with the statistic value based on the resonance frequency or the spring constant of the tire at each of a plurality of time points.
That is, according to the invention, in estimating the tire air pressure, by selectively switching to estimation based on the resonance frequency or the spring constant by the first estimating component having high estimation accuracy when the vehicle is operating at low and mid speeds, and to estimation based on the dynamic load radius by the second estimating component having high estimation accuracy when the vehicle is operating at high speed, regardless of the vehicle travel speed, high estimation accuracy is achieved. As a parameter for switching, the statistic value based on the resonance frequency or the spring constant of the tire in actually operating the vehicle is used. Therefore, regardless of various elements to be mounted to the vehicle, the tire air pressure can be estimated with high accuracy.
In this way, according to the apparatus for estimating tire air pressure of the first embodiment of the invention, estimation of the air pressure by the first estimating component for estimation based on the resonance frequency or the spring constant of the tire, and estimation of the air pressure by the second estimation component based on the dynamic load radius, are used selectively accordance with the statistic value based on the resonance frequency or the spring constant of the tire at each of the plurality of time points. Therefore, regardless of various elements to be mounted to the vehicle, the tire air pressure can be estimated with high accuracy from operation at low to high speeds.
As the statistic value according to the first embodiment of the invention, there can be applied any one of a variance value, a standard deviation value, bias and a kurtosis of the resonance frequencies or the spring constants at the plurality of time points.
FIG. 10A and FIG. 10B show an example of histograms of resonance frequencies at a plurality of time points extracted based on a wheel speed signal. FIG. 10A shows an example of a case in which vehicle speed is a mid speed of xe2x80x9caxe2x80x9d km/h and FIG. 10B shows an example of case of in which the vehicle speed is high speed of xe2x80x9cbxe2x80x9d km/h.
As shown by FIG. 10A, the histogram of the resonance frequency when the vehicle speed is the mid speed of xe2x80x9caxe2x80x9d km/h substantially forms a shape of a normal distribution and a variance value of the resonance frequencies in this case has been xe2x80x9cAxe2x80x9d. In contrast thereto, as shown by FIG. 10B, the histogram of the resonance frequency when the vehicle speed is the high speed of xe2x80x9cbxe2x80x9d km/h, forms an irregular state and a variance value of the resonance frequency in this case has been xe2x80x9cBxe2x80x9d, which is larger than A as mentioned above. As described above, the spring constant of the tire is an index having a high correlation with the resonance frequency. Therefore, a histogram of the spring constant behaves similar to the case for the resonance frequency.
Therefore, by selectively switching between estimation by the first estimating component and the second estimating component in accordance with the variance value of the resonance frequency (spring constant) provided based on the wheel speed signal, the air pressure can be estimated with high accuracy.
Also a standard deviation value having a correlation with the variance value can be dealt with similar to the case of the variance value.
Bias (a degree of shifting the vertex of a normal distribution as shown by FIG. 10A in the left and right directions) or a kurtosis of the resonance frequency (a pointed degree of the normal distribution state as shown by FIG. 10A) can also be considered as the index of representing a degree of accuracy of the extracted resonance frequency. Therefore, also the bias and the kurtosis are applicable as indices for selectively switching of estimation by the first component and second estimating components.
According to a second preferred embodiment of the invention, there is provided a tire air pressure estimating apparatus comprising an extracting component for extracting a resonance frequency or a spring constant of a tire based on a wheel speed signal including frequency components of vibration of the tire in operating a vehicle, a first estimating component for estimating tire air pressure based on the resonance frequency or the spring constant of the tire extracted by the extracting component; a deriving component for deriving a dynamic load radius of the tire based on the wheel speed signal; a second estimating component for estimating the tire air pressure based on the dynamic load radius derived by the deriving component; a wheel speed deriving component for deriving a wheel speed based on the wheel speed signal; a estimation switching component for estimating the tire air pressure by the first estimating component when the wheel speed derived by the wheel speed deriving component is less than a predetermined value and estimating the tire air pressure by the second estimating component when the wheel speed at least equals the predetermined value, and a setting component for setting the predetermined value based on the resonance frequency or the spring constant of the tire.
The resonance frequency or the spring constant of the tire is extracted based on the wheel speed signal including the frequency components of vibration of the tire in operating the vehicle, and based on the extracted resonance frequency or the extracted spring constant of the tire, the tire air pressure is estimated. As a method of estimating the tire air pressure by the first estimating component, similar to the first embodiment of the invention, there can be applied all the existing methods capable of estimating the tire air pressure based on the resonance frequency or the spring constant of the tire, such as estimating methods described in such publications as, for example, Japanese Patent Registration No. 2,836,652, JP-A No. 9-2,031, JP-A No. 6-297,923 and JP-A No. 8-219,920.
According to the second embodiment of the invention, the dynamic load radius of the tire is derived based on the wheel speed signal. The tire air pressure is estimated by the second estimating component based on the derived dynamic load radius. Also as methods of estimating tire air pressure by the second estimating component, there can be applied all the existing methods capable of estimating the tire air pressure based on the dynamic load radius of the tire such as, for example, a estimating method described in JP-A No. 9-2,031.
Further according to the second embodiment of the invention, the wheel speed is derived based on the wheel speed signal by the wheel speed deriving component. Using the estimation switching component, operation is switched to estimate the tire air pressure by the first estimating component when the derived wheel speed is less than the predetermined value. Operation is switched to estimate the tire air pressure by the second estimating component when the wheel speed at least equals the predetermined value.
That is, according to the invention, in estimating the tire air pressure, by selectively switching the estimation based on the resonance frequency or the spring constant by the first estimating component having high estimation accuracy when the mounted vehicle is operating at low to mid speeds, and the estimation based on the dynamic load radius by the second estimating component having high estimation accuracy when the mounted vehicle is operating at high speed, regardless of vehicle travel speed, high estimation accuracy is achieved.
Here, According to the invention, by the setting component, the predetermined value for a threshold switching by the estimation switching component, is set based on the resonance frequency or the spring constant of the tire in actually operating the vehicle. Thereby, regardless of various elements of the mounted vehicle, the tire air pressure can be estimated with high accuracy.
In this way, according to the apparatus for estimating tire air pressure of the second embodiment of the invention, operation is switched to estimate the tire air pressure by the first estimating component for estimating the tire air pressure based on the resonance frequency or the spring constant of the tire when the vehicle speed is less than the predetermined value. Operation is switched to estimate the tire air pressure by the second estimating component for estimating the tire air pressure based on the dynamic load radius when the vehicle speed at least equals the predetermined value. The predetermined value is set based on the resonance frequency or the spring constant of the tire and accordingly, regardless of various elements to be mounted to the vehicle, the tire air pressure can be estimated with high accuracy from operating at low speed high speeds.
It is preferable that the setting component according to the second embodiment of the invention, sets the predetermined value in accordance with a statistic value based on the resonance frequency or the spring constant of the tire at each of a plurality of time points. In this case, as the statistic value, by reason similar to that of the first embodiment of the invention, there is applicable any one of a variance value, a standard deviation value, bias and a kurtosis of the resonance frequency or the spring constant at each of the plurality of time points.
In accordance with a third embodiment of the present invention, there is provided a method for estimating tire air pressure. The method includes: (a) extracting a resonance frequency or a spring constant of the tire based on a wheel speed signal including frequency components of vibration of a tire in operating a vehicle; (b) estimating the tire air pressure based on the resonance frequency or the spring constant of the tire extracted; (c) deriving a dynamic load radius of the tire based on the wheel speed signal; (d) estimating the tire air pressure based on the dynamic load radius derived; and (e) selectively switching estimation of the air pressure by step (b) and a estimation of the air pressure using said steps of estimating tire air pressure based on resonance frequency or a spring constant, and the dynamic load radius in accordance with a statistic value based on the resonance frequency or the spring constant of the tire at a plurality of time points.
In accordance with a fourth embodiment of the present invention, there is provided a method for estimating tire air pressure. The method includes:(a) extracting a resonance frequency or a spring constant of the tire based on a wheel speed signal including frequency components of vibration of the tire in operating a vehicle; (b) estimating tire air pressure based on the resonance frequency or the spring constant of the tire extracted; (c) deriving a dynamic load radius of the tire based on the wheel speed signal; (d) estimating the tire air pressure based on the dynamic load radius derived; (e) deriving a wheel speed based on the wheel speed signal; (f) switching from estimation of the tire air pressure by the step of estimating the tire air pressure based on the resonance frequency or the spring constant value when the wheel speed derived is less than a predetermined value and to estimation of the tire air pressure by the step of estimating the tire air pressure based on the dynamic load radius when the wheel speed at least equals the predetermined value; and (g) setting the predetermined value based on the resonance frequency or the spring constant of the tire.